Selective extraction of specific anions from a solution can be useful in several different contexts, but has been limited in practice by the inability of anion extraction to overcome the Hofmeister bias. In liquid-liquid anion extraction, the Hofmeister bias is a thermodynamic bias that favors the extraction of charge diffuse, weakly hydrated anions from water into an organic medium. Therefore, if the anion which is desired to be extracted is in competition with an anion which is more charge diffuse and weakly hydrated, selective extraction of the desired anion must overcome the Hofmeister bias.
One example of a system where two anions are in aqueous solution, and it is desirable to remove one of the anions selectively from the solution, is nuclear waste, where both nitrates and sulfates are in solution. Sulfate separation is desirable to improve vitrification of the nuclear waste, but the Hofmeister bias greatly favors the extraction of nitrate from the solution instead of sulfate.
Sulfate separation is also desirable for waste streams to avoid affecting natural biogeochemical cycles and the metabolisms of living organisms. The uncontrolled release of anthropogenic sulfate can have a significant impact on the environment, such as by increasing Earth's albedo or leading to eutrophication of rivers or lakes.
Extraction of other anions from a liquid solution in contravention to the usual Hofmeister bias is also desirable in certain liquid handling and separations situations. Additional anions for preferential extraction may include phosphate, arsenate, chloride, carbonate, fluoride, selenate, selenite, sulfite, chromate, pyrophosphate, arsenite, and tetrafluoroberyllate.
Phosphate, along with sulfate, leads to eutrophication of lakes and rivers, therefore methods for the specific removal of phosphates, in addition to sulfates, from water entering lakes and rivers are highly sought after, such as removal of phosphates and sulfates widely used in fertilizer from agricultural runoff. Chloride has also been found to accumulate in lakes due to the use of large amounts of road-salt (NaCl, CaCl2) in the winter, and cause adverse environmental effects. Therefore, preferential removal of chloride from such environments is desirable. Anions such as chromate, arsenate, arsenite, selenate, selenite, and tetrafluoroberyllate are highly toxic, therefore they need to be removed very efficiently from waste waters at industrial and mining sites, or water purification plants.